Chapter 6: Bones and Bone Structure – Study Notes for Anatomy & Physiology (BIO 141)

Overview of the Skeletal System

Major Functions of the Skeletal System

The skeletal system provides the structural framework for the human body and serves several essential physiological functions.

• Support: Bones support the body and provide attachment points for muscles.

• Protection: Bones protect vital organs, such as the brain (skull), heart and lungs (rib cage).

• Leverage: Bones act as levers to facilitate movement in response to muscle contraction.

• Storage of Minerals and Lipids: Bones store minerals (especially calcium and phosphate) and lipids (yellow bone marrow).

• Blood Cell Production: Red bone marrow produces red blood cells, white blood cells, and other blood elements.

Additional info: The skeletal system also plays a role in maintaining homeostasis of mineral ions and contributes to endocrine regulation.

• Bone Tissue Composition

Minerals and Lipids in Bone

Bones are reservoirs for minerals and lipids, which are essential for various physiological processes.

• Minerals: Inorganic ions (e.g., calcium, phosphate) contribute to the osmotic balance and serve as cofactors for enzymes.

• Calcium: The most abundant mineral in the body, stored primarily in bone tissue.

• Lipids: Stored in bone as yellow bone marrow, which serves as an energy reserve.

Classification of Bones

Types of Bones by Shape

Bones are classified according to their shapes, which relate to their functions and locations in the body.

• Sutural Bones (Wormian Bones): Small, flat, irregularly shaped bones found between the flat bones of the skull. Their borders are like pieces of a jigsaw puzzle.

• Irregular Bones: Complex shapes with short, flat, notched, or ridged surfaces. Examples include vertebrae, pelvic bones, and some skull bones.

• Short Bones: Boxlike appearance. Examples include carpal (wrist) and tarsal (ankle) bones.

• Flat Bones: Thin, parallel surfaces. Found in the roof of the skull, sternum, ribs, and scapulae. Provide protection and a large surface area for muscle attachment.

• Long Bones: Relatively long and slender. Found in the arms, legs, palms, soles, fingers, and toes. The femur is the largest and heaviest bone.

• Sesamoid Bones: Usually small, round, and flat. Found near joints (e.g., patella/kneecap). Not everyone has sesamoid bones at every possible location.

Table: Bone Types and Examples

Bone Type	Description	Examples
Sutural (Wormian)	Small, flat, irregularly shaped; found in skull sutures	Between cranial bones
Irregular	Complex shapes	Vertebrae, pelvis, some skull bones
Short	Boxlike, small	Carpals, tarsals
Flat	Thin, parallel surfaces	Skull, sternum, ribs, scapulae
Long	Long and slender	Femur, humerus, radius, ulna
Sesamoid	Small, round, flat; develop in tendons	Patella

Bone Markings

Types and Functions of Bone Markings

Bones have characteristic surface features called markings, which serve as sites for muscle, tendon, and ligament attachment, or as passageways for nerves and blood vessels.

• Openings: Allow passage of nerves and blood vessels (e.g., foramen, canal, fissure).

• Projections: Sites of muscle and ligament attachment (e.g., tuberosity, crest, spine).

• Depressions: Accommodate other structures (e.g., fossa, groove).

Additional info: Bone markings are essential for anatomical orientation and clinical identification.

Bone Structure and Organization

Compact vs. Spongy Bone

Bones are composed of two main types of tissue: compact bone and spongy bone, each with distinct structure and function.

• Compact Bone: Dense, solid bone that forms the outer layer of all bones. Organized into osteons (Haversian systems) with concentric lamellae around a central canal.

• Spongy Bone: Less dense, found at the ends of long bones and inside flat bones. Composed of a network of trabeculae, which provide structural support and house bone marrow.

Table: Comparison of Compact and Spongy Bone

Feature	Compact Bone	Spongy Bone
Location	Outer layer of bones	Ends of long bones, inside flat bones
Structure	Osteons, concentric lamellae	Trabeculae, meshwork
Function	Strength, protection	Lightweight, supports marrow

Bone Cells and Their Functions

Types of Bone Cells

Bone tissue contains several specialized cell types, each with a unique role in bone formation, maintenance, and remodeling.

• Osteogenic Cells: Stem cells that differentiate into osteoblasts; important for bone growth and repair.

• Osteoblasts: Cells that synthesize new bone matrix through osteogenesis/ossification.

• Osteocytes: Mature bone cells that maintain bone tissue; reside in lacunae and communicate via canaliculi.

• Osteoclasts: Large, multinucleated cells that resorb (break down) bone matrix; important for calcium regulation.

Additional info: Osteoclasts are derived from the same stem cells as monocytes and macrophages.

Bone Development and Growth

Ossification Processes

Bone formation occurs through two main processes: endochondral ossification and intramembranous ossification.

• Endochondral Ossification: Bone replaces existing cartilage; responsible for the formation of long bones.

• Intramembranous Ossification: Bone develops directly from mesenchyme or fibrous connective tissue; forms flat bones.

Growth: Bones grow in length (interstitial growth) and width (appositional growth).

Bone Remodeling and Homeostasis

Remodeling Mechanisms

Bone remodeling is a continuous process involving the coordinated activity of osteocytes, osteoblasts, and osteoclasts.

• Osteoblasts: Add new bone matrix.

• Osteoclasts: Remove bone matrix.

• Osteocytes: Maintain bone matrix and regulate mineral content.

Remodeling adapts bone architecture to mechanical stress and maintains mineral homeostasis.

Hormonal Regulation of Bone

Key Hormones Affecting Bone Growth and Maintenance

Several hormones regulate bone growth, remodeling, and mineral balance.

• Growth Hormone (GH): Stimulates bone growth.

• Sex Hormones (Estrogen, Testosterone): Promote osteoblast activity and epiphyseal closure.

• Parathyroid Hormone (PTH): Increases blood calcium by stimulating osteoclasts.

• Calcitonin: Lowers blood calcium by inhibiting osteoclasts.

• Thyroid Hormones: Stimulate bone growth and matrix synthesis.

Bone Fractures and Repair

Types of Fractures

Fractures are classified by the nature and location of the break.

• Displacement: Nondisplaced or displaced

• Completeness: Complete or incomplete

• Orientation: Linear or transverse

• Penetration: Open (compound) or closed (simple)

Steps in Fracture Repair

Bone repair involves four main steps:

1. Hematoma formation

2. Fibrocartilaginous callus formation

3. Bony callus formation

4. Bone remodeling

Bone Disorders

Osteopenia and Osteoporosis

Bone disorders can compromise normal function and increase fracture risk.

• Osteopenia: Inadequate ossification of bone; decreased bone mass.

• Osteoporosis: Severe reduction in bone mass, often due to decreased sex hormones; more common in older females.

Additional info: Osteoporosis can be accelerated by cancers producing osteoclast-activating factors.